ENH: coreg opacity slider

[GuillaumeFavelier]

This PR is extracted from #10224:

• adds a slider for the head surface opacity (semi-automatic behaviour)

[hoechenberger]

Generally I would approve, BUT the default of 0.95 doesn't work for me (in fact, not even 0.99 works for me). I'm pretty convinced there's a bug somewhere, probably in pyvista. As soon as even the tiniest bit of transparency is enabled, everything gets WAY too transparent and the usual "the back of the head looks like the front" happens for me. It's almost like a binary switch between "not transparent at all" and "full-blown transparency" (which, admittedly, can be slightly adjusted using the slider later on) I've never seen such behavior in any software I've used. This just cannot be right

@GuillaumeFavelier I can demonstrate to you what I mean later – out for a coffee now

[larsoner]

Easy enough to reproduce:

Peek.2022-01-26.10-16.mp4

Enabling depth peeling (just pushed) fixes it for me, but 0.95 seems way too opaque:

Peek.2022-01-26.10-18.mp4

@hoechenberger want to try on my commit and see what the ideal transparency value is to you? To me 0.8 works well because you can clearly see the bad black points:

[hoechenberger]

@larsoner Perfect!! I changed the default to 0.8 and pushed.

@GuillaumeFavelier WDYT?

[GuillaumeFavelier]

This is a way better transition 💯

[GuillaumeFavelier]

Shall we update mne/gui/tests/test_coreg_gui.py and mne/gui/__init__.py too ?

diff --git a/mne/gui/__init__.py b/mne/gui/__init__.py
index 53719bf5d..ba9a1b00c 100644
--- a/mne/gui/__init__.py
+++ b/mne/gui/__init__.py
@@ -151,7 +151,7 @@ def coregistration(tabbed=False, split=True, width=None, inst=None,
         advanced_rendering = \
             config.get('MNE_COREG_ADVANCED_RENDERING', 'true') == 'true'
     if head_opacity is None:
-        head_opacity = config.get('MNE_COREG_HEAD_OPACITY', 0.95)
+        head_opacity = config.get('MNE_COREG_HEAD_OPACITY', 0.8)
     if head_inside is None:
         head_inside = \
             config.get('MNE_COREG_HEAD_INSIDE', 'true').lower() == 'true'
diff --git a/mne/gui/tests/test_coreg_gui.py b/mne/gui/tests/test_coreg_gui.py
index cfdf6dc84..386f7f979 100644
--- a/mne/gui/tests/test_coreg_gui.py
+++ b/mne/gui/tests/test_coreg_gui.py
@@ -185,7 +185,7 @@ def test_coreg_gui_pyvista(tmp_path, renderer_interactive_pyvistaqt):
     assert coreg._mark_inside
     assert_allclose(
         coreg._head_opacity,
-        float(config.get('MNE_COREG_HEAD_OPACITY', '0.95')))
+        float(config.get('MNE_COREG_HEAD_OPACITY', '0.8')))
     assert coreg._project_eeg == \
         (config.get('MNE_COREG_PROJECT_EEG', '') == 'true')
     assert coreg._hpi_coils

[larsoner]

Please do @GuillaumeFavelier

[hoechenberger]

CI errors are due to download problems. We should try to restart in a few minutes (or in an hour??) or so

[larsoner]

Thanks @GuillaumeFavelier !

[hoechenberger]

🥳